The present invention relates in general to data processing systems, and in particular, to a triode assembly for a cold cathode field emission device.
Carbon-based cold cathodes are relatively flat substrates on which a carbon film is deposited. If the carbon film is deposited with the correct parameters, this film will emit electrons when the surface of the cathode is subjected to an electric field that is normal to the carbon film surface. These electric fields are generally on the order of 3-10 volts/micron for current densities of 1-10 mA/cm2. For many applications, the average current density must be controlled very accurately and very rapidly. This requires rapid control of the field that is applied. The field is applied by switching a voltage across a gap between the cathode and another electrode. The smaller the gap between the cathode and the switching electrode, the smaller the voltage required to switch the emission properties of the cathode on and off. Small switching voltages allow cheaper and more efficient drivers that are needed to switch the voltages on the device.
A typical configuration for a display device is to have three electrodes: a cathode, and anode, and a grid. The grid electrode is typically the switching electrode for switching the current from the cathode on and off.
For Spindt-type microtip technology, the cathode and grid electrodes are integrated on the same substrate. Both the tip and the electrode are engineered so the electron source (top of the tip) is placed very precisely in the grid opening. This self-aligned process optimizes the performance of the grid and cathode electrodes, allowing the grid electrode to operate at nearly 100 percent efficiency. For a non-engineered cathode, such as carbon cold cathodes, the electron sources (emission sites) are randomly placed on the cathode. Thus, to make a triode assembly using a carbon cold cathode, engineers have resorted to two techniques, neither of which is ideal.
The first technique, referring to FIG. 1, a carbon cold cathode is produced first by depositing an emitter 102 on a substrate 101. Then a grid electrode 104 is mounted to the cathode 101 with spacers 103 between the grid 104 and the cathode 101, such that the grid electrode 104 is suspended over the active carbon areas 102 of the cathode 101. The grid 104 is a perforated metal foil. If conducting spacers are used, they must be placed on an insulating part of the cathode. Even if insulating spacers are used, they must be placed carefully in order to avoid charge up and arcing between the cathode and the grid. Generally, this approach leads to large gaps between the cathode and the grid electrode, which means that the driving voltages tend to be high. Furthermore, this approach is not very efficient since a large percentage, 50% or higher, of the electron current from the cathode can go to the grid electrode. This warms the grid electrode unnecessarily, which decreases the device efficiency and may also lead to mechanical difficulty in operating the device.
In a second technique, referring to FIGS. 2A, 2B and 2C, a cathode/grid integrated grid structure is first produced on a single substrate, then a carbon film 205 is grown thereon. For example, in FIGS. 2A and 2B, a substrate 201 has deposited and patterned thereon an insulating layer 202 and a metal layer 203, which acts as the grid layer. Holes 204 are thus formed within the grid layers. Thereafter, in FIG. 2C, a carbon film 205 is deposited on the entire structure, so that the carbon material 205 not only deposits within the holes 204, but also on the sidewalls and on the top of the grid structure, thus potentially compromising the structure and making it inoperable, as described below.
Generally, it has not been possible to make an integrated cathode/grid assembly after the carbon film has been deposited since the processes that are used to make the structure severely degrade the performance of the carbon film. Making the structure first and then depositing the carbon film, as illustrated in FIGS. 2A-2C, also has problems. The structure must survive the high temperatures and extremely reactive environments needed for carbon deposition. Generally, this leads to structure failure, either because the structure physically falls apart, or because the insulating layer 202 used between the cathode layer 201 and the grid layer 203 becomes conductive as a result of exposure to the extreme environments.
As a result of the foregoing, there is needed in the art an improved triode assembly for a carbon cold cathode.
The present invention addresses the foregoing needs by providing for a grid assembly that includes a conducting layer and an insulating layer that is then placed directly onto a carbon cathode. The carbon cathode is prepared separately from the grid assembly so the grid assembly is not exposed to the extreme environment required to make the carbon cathode. In this approach, an insulating layer is placed on a conducting foil. It is also possible to place a conducting layer on an insulating foil and achieve the same result. The insulating and conducting layer are then placed directly onto the cathode and bonded to the cathode using adhesives.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.